There is well-known a technique of grafting natural rubber with a vinyl compound, and MG latex and the like are already produced and are put into a practical use as an adhesive or the like (e.g. JP-A-2000-319339, JP-A-2002-138266). In this grafted natural rubber, however, a greater amount of the vinyl compound (20-50% by weight) is grafted as a monomer for changing the characteristics of natural rubber itself, so that the physical properties inherent to natural rubber (viscoelasticity, stress-strain curve in a tensile test or the like) is largely changed. Also, when the grafted natural rubber is compounded with carbon black and/or silica, the serious viscosity rise is caused to decrease the processability. Furthermore, as a greater amount of the other monomer is introduced into the molecular chain of natural rubber, the physical properties become entirely different from those inherent to natural rubber. Therefore, there is caused a problem that such a grafted natural rubber can not quite utilize the conventional methods making the beat use of the physical properties inherent to natural rubber.
As a technique for improving the reinforcing property and affinity of a polymer with a filer such as carbon black, silica or the like, there are developed a terminal modification, a copolymerization with a monomer containing a functional group and so on in the field of synthetic rubbers. On the other hand, natural rubber is used in a greater amount owing to the excellent physical properties, but there is not known a technique that natural rubber itself is largely improved so as to make the reinforcing property and affinity to the filler equal to those of the above modified synthetic rubber.
On the other hand, there are proposed a technique wherein an ester of a polyvalent alcohol with an unsaturated carboxylic acid, an organic compound having an unsaturated bond, or a vinyl monomer is compounded with natural rubber to conduct graft polymerization, a technique of epoxidizing natural rubber, and so on (e.g. JP-A-5-287121, JP-A-6-329702, JP-A-9-25468, JP-A-2002-348559). Particularly, the former technique is not a technique aiming at the improvement of the reinforcing property and affinity to the filler such as carbon black, silica or the like, but is a technique of largely changing the physical properties inherent to natural rubber. In these techniques, however, there are problems that an odor is generated in the mastication or the like, and a freeze resistance is poor in such an area that ambient temperature becomes not higher than 10° C. in winter season. Furthermore, it can not be said that these techniques develop the sufficient improving effects.